1. Field of the Invention
The current invention describes finned tubes used for heat transfer, such as the tubes used in shell and tube heat exchangers.
2. Description of the Related Art
Finned tubes have been used for heat transfer for many years. Heat flows from hot to cold, so heat transfer is accomplished by conducting heat from a warmer material to a cooler material. There is also heat given off when a material condenses from a vapor to a liquid, and heat is absorbed when a liquid vaporizes or evaporates from a liquid to a vapor. When finned tubes are used for heat transfer, the warmer material is on either the inside or the outside of the tube and the cooler material is on the other side. Usually the tube allows for the transfer of heat without mixing the warmer and cooler materials.
For cooling purposes, a cooling medium can be a liquid such as cooling water flowing through a shell and tube heat exchanger, or it can be a gas such as air blown over a finned tube. Similarly, a heating medium is usually either a liquid or a gas. Finned tubes are sometimes used instead of relatively smooth tubes because finned tubes tend to increase the rate of heat transfer. Therefore, a smaller heat exchanger with finned tubes may be able to transfer as much heat in a given application as a larger heat exchanger with relatively smooth tubes. The design of finned tubes affects the rate of heat transfer and sometimes the tubes are designed differently for specific heat transfer applications. For example, finned tubes used for condensation tend to have different designs than finned tubes used for evaporation.
Examples of the prior art include finned tubes with helical ridges formed on an inner surface of the tube and fins formed on an outer surface of the tube. A channel is defined by adjacent fins on the tube outer surface, and this channel can have a curved, “U” shaped bottom or the channel can have a flat bottom. When used as condensing tubes with the vapor condensing on the outside of the tube and coolant flowing inside the tube, the channels tend to become filled with liquid condensate. The liquid condensate serves to insulate the tube and restrict the cooling needed for further condensation. The flat bottom is preferred because condensate tends to spread out along the bottom of the flat channel instead of creeping up the sides of the fins. This leaves more surface area on the fins free of condensate, which enhances heat transfer.
Finned tubes also have had breaks formed in the fins so condensate flowing within a channel between two fins could flow through a break and enter a different channel. Other finned tubes have had the outer portion of the fin bent over so that a bend is formed part of the way between a base of the fin and a top of the fin. This creates additional angles in the fin which tends to cause the tube to shed liquid condensate more rapidly. When liquid condensate is shed from a tube more rapidly, it tends to enhance heat transfer. Other fins have had notches formed in the fin tip with peaks defined between the notches. In some cases the peaks are bent over to form a curl shape. This again increases curvature and angles in the fin and thereby tends to cause the tube to shed liquid condensate more rapidly.
Some finned tubes are produced by attaching fin material to a relatively smooth tube so the fins are not formed from the material of the tube body. This increases the area available for heat transfer, which does improve heat transfer rates, but the interface between the fin and the tube does cause some resistance to heat flow. The fins attached to the tube can extend radially from a tube axis so they stand straight up from the tube, but they can also be curved or bent in various ways to improve heat transfer.
Some tubes are designed for evaporation on the tube outer surface. For example, fins can be formed on the tube outer surface, and then notches can be depressed into the fin top. Next, the fin is bent over so the fin top touches the adjacent fin such that the bent fin forms a roof over the channel between the two adjacent fins. This produces a cavity which is mostly enclosed between the tube outer surface and two adjacent fins. The notches in the fin top allow liquid to flow into the cavity and vapor to escape from the cavity. There are many designs of finned tubes in existence, but changes which improve heat transfer are still possible.